A GEPON system is comprised of a station optical line terminating device (OLT: Optical Line Terminal) disposed in a center station, an optical divider for dividing a transmission line into up to 32 lines, and subscriber optical network terminating devices each disposed in a member's house.
In the GEPON system, a wavelength of 1,310 nm is assigned to an uplink digital data signal which is transmitted from each subscriber optical network terminating device to the station optical line terminating device, a wavelength of 1,490 nm is assigned to a downlink digital data signal (including a digital sound signal) transmitted from the station optical line terminating device to each subscriber optical network terminating device, and a wavelength of 1,550 nm is assigned to a downlink video signal (including an analog video signal).
Furthermore, it has been examined conventionally with an eye towards future technological innovation that in the GEPON system, a wavelength band of 1,565 nm or longer is assigned as the wavelengths of digital data signals which are transmitted at a transmission rate of 10 gigabits/second. Furthermore, the wavelength band of 1,565 nm or longer is used also as a light wavelength band for examining a disconnection of a line optical fiber connected between the station optical line terminating device and each subscriber optical network terminating device.
Thus, in the GEPON system, a wavelength division multiplexing method (WDM: Wavelength Division Multiplexing) of assigning a plurality of wavelengths is used to carry out single-core bidirectional optical communications via which an uplink wavelength and a downlink wavelength are transmitted by using a single optical fiber.
However, in the GEPON system, it is necessary to provide a light wavelength band limiting filter for interference prevention for the purpose that each subscriber optical network terminating device does not cause interference (cross talk) between the wavelength of the downlink digital data signal and the wavelength of the downlink video signal.
More specifically, the optical transmission/reception module mounted in each subscriber optical network terminating device needs to include a light wavelength band limiting filter for interference prevention.
Typically, the light wavelength band limiting ability of the light wavelength band limiting filter greatly depends on the light incidence angle to the filter.
More specifically, when the number of components having different light incidence angles increases, the light wavelength band limiting filter exhibits a combined light wavelength band limiting ability according to the components having different light incidence angles.
Particularly, in a case in which the wavelength interval between received wavelength bands is narrow, while a light signal having a wavelength band which is desired originally to pass through the filter is blocked, a wavelength band which is desired to be blocked is allowed to pass through the filter. The light wavelength band limiting filter thus cannot sufficiently exhibit its light wavelength band limiting ability.
Therefore, a light wavelength band limiting filter having a high required ability to prevent interference (cross talk) of an optical reception signal is used in a collimating optical system which can reduce light incidence angle components in many cases, but is used rarely in a diffusing optical system. However, the collimating optical system has a complicated structure, as will be mentioned below.
The angle of incidence of light to a light wavelength band limiting filter depends on a displacement caused by an installation angle with respect to an optical fiber a displacement caused by an installation angle with respect to a wavelength division multiplexing filter (WDMF: Wavelength Division Multiplexing Filter), a displacement caused by an installation angle of the light wavelength band limiting filter itself, etc.
Because it is impossible to measure the angle of incidence of light to the light wavelength band limiting filter, it is necessary to design the optical transmission/reception module in consideration of the amount of angular displacement within the limits of design assurance in the structural design.
Although in a GEPON system an optical transmission/reception module mounted in a subscriber optical network terminating device needs to include a light wavelength band limiting filter for interference prevention, as mentioned above, an optical transmission/reception module disclosed by patent reference 1 mentioned below carries out demultiplexing and multiplexing of light signals having a plurality of wavelengths by using a wavelength division multiplexing filter to implement single-core bidirectional optical communications.
However, because a lens coupling optical element is only connected between the wavelength division multiplexing filter and the optical fiber in this optical transmission/reception module, the optical transmission/reception module cannot be applied to a GEPON system in which a light wavelength exists adjacent to each of the light wavelength of a downlink digital data signal and the light wavelength of a signal for video image.
In order to maintain the light wavelength band limiting ability of a light wavelength band limiting filter to prevent interference (cross talk) of an optical reception signal, collimating optical equipment or the like in which the inside of an optical transmission/reception module is replaced by a collimating optical system (collimate optical system) is disposed and the light wavelength band limiting filter is used in the collimating optical system. As a result, the light wavelength band limiting filter can exhibit the light wavelength band ability.
A problem with the collimating optical system is, however, that the component count increases and the collimating optical system has a complicated structure.
Furthermore, in a diffusing optical system, if the angle of incidence of light to a light wavelength band limiting filter can be managed with a high degree of accuracy, the light wavelength band limiting ability of the light wavelength band limiting filter can be improved.
However, in order to adjust the angle of incidence of light, an advanced alignment technique is required.